1. Field of the Invention
THe present invention relates to apparatus and methods for quantifying the irregularities in interference fringe patterns commonly encountered in optical metrology. More particularly, the invention relates to apparatus for use in conjunction with a standard monochrome closed-circuit television system for the generation, display, and manipulation of geometrical patterns upon the screen of the video monitor for the purpose of quantifying the irregularities in an interference pattern displayed on the screen of the video monitor.
2. The Prior Art
Interferometric testing has long been used in optical metrology. The advent of the laser has not only made interferometers more convenient to use but has also extended their range of application. Interferometry is used as a tool in the fabrication, final testing, and system slignment, see for example, C. Zanoni, "Interferometry," The Optical Industry and Systems Directory Encyclopedia, v. 2, E137 (1977).
For most interferometry the output of the test is either an interference fringe pattern or an interferogram. The type of pattern is usually determined by the particular measurement configuration even though in some instances the errors in the article under test can dictate the type of pattern. Nevertheless, the quantitative reduction of an interference fringe pattern is usually based on ascertaining the fractional departure of the interference fringe pattern from some ideal, best-fitting pattern. The denominator of the fractional departure is usually the average spacing between a pair of fringes in the ideal pattern.
The quantitative usefulness of an interference pattern is dependent upon having a method of data extraction and reduction. Interference pattern reduction can range in complexity from a simple visual evaluation to an elaborate reduction of the data extracted by an automatic microdensitometer with a large computer, see, for example, R. A. Jones and P. L. Kadakia, "An Automated Interferogram Technique," Appl. Opt. 7, 1477 (1968). In between these extremes there are a great variety of means for hand reduction, see, for example, R. Berggren, "Analysis of Interferograms," Optical Spectra, December, 1970, p. 22.
A common technique for reducing interferograms by hand uses a device comprised of a hinged parallelogram with a number of equally spaced, hinged members to produce a grid of parallel, equally spaced straight lines of variable spacing. While this technique is useful for a casual evaluation, it is time consuming and tedious to extract quantitative results.
The reduction of interferograms by hand is further complicated when there is a power or a focus error which produces a curvature in the fringes. The bull's-eye pattern with non-uniformly spaced concentric circles is a familiar example. Hand reduction of these complex fringe patterns is extremely tedious and time consuming.
Sophisticated, expensive interferometers have been designed and built for high precision, automatic reduction of interferograms, e.g., one such instrument is disclosed in Gallager et al., U.S. Pat. No. 3,694,088, Sept. 26, 1972. Another sophisticated digital interferometer is discussed in J. H. Bruning, et al., "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses," Appl. Opt. 13, 2693 (1974).
In many industrial applications, it is desirable to reduce interferograms quickly in a simple, economic manner. For example, in the manufacture of high precision, high volume optical components, interferograms and interferometer interference patterns must be measured in large numbers and at high speed with affordable instrumentation.
While the prior art methods are useful for some applications, they cannot be used in many industrial applications similar to the one mentioned above.